Larvae of green mussel, Perna viridis, were monitored in the culture area of Si Racha Bay, Chonburi Province of Thailand to estimate the best time to collect spat needed for suspended culture. Green mussel larvae were collected by plankton net every two weeks from July 1996 to June 1997. Green mussel larvae occurred almost throughout the year, but the highest numbers appeared in the beginning of March 1997, and two other peaks in September to October 1996 and December l997.